The disclosed subject matter relates generally to telecommunications and, more particularly, to a method and apparatus for dynamically scheduling ringing signals.
In communications systems, particularly telephony, it is common practice to transmit signals between a subscriber station and a central switching office via a two-wire bi-directional communication channel. A line card generally connects the subscriber station to the central switching office. A line card typically includes a plurality of subscriber line interface circuits (SLIC) as well as one or more subscriber line audio-processing circuits (SLAC). The functions of the line card range from providing a power supply to performing wake-up sequences of circuits to allow communications to take place.
Subscriber line interface circuits (SLICs) have been developed to provide an interface between a low voltage signal path in a telephone central office and a high-voltage telephone subscriber line. The SLIC provides functions such as off hook detection, ringing signal generation, and battery feed to the subscriber line. The subscriber line consists of a telephone transmission line, including two conductors referred to as A and B or tip and ring, and the subscriber telephone equipment coupled across the tip and ring conductors (i.e., the load). The subscriber line and the subscriber telephone equipment are also referred to as a subscriber loop.
The SLIC provides power from the telephone central office to the subscriber line in response to a received battery voltage. The battery voltage is a DC voltage supplied to the SLIC to power the SLIC and the subscriber line. A typical value of the battery voltage is −48 VDC. The battery voltage has a value generally in the range −20 to −60 VDC. The SLIC supplies a DC current at the battery voltage to the subscriber line. Superimposed on the DC current are AC signals of audio frequency by which information is conveyed between the subscriber and the central office. The battery voltage is generated at the central office, either by a depletable energy storage device such as a battery or by a DC generator, for supply to the SLIC. In a central office, one battery or DC generator supplies the battery voltage to many SLICs and their associated subscriber loops.
In many modern applications, a SLIC is located remote from the central office, relatively close to the subscriber telephone equipment and coupled to the subscriber telephone equipment by a relatively short subscriber line. For example, in fiber in the loop (FITL) applications, the SLIC is located in the same city neighborhood as the subscriber telephone equipment and is coupled to the subscriber telephone equipment by tip and ring conductors no more than a few hundred feet in length. The SLIC or an associated circuit receives optical signals from the central office over an optical fiber and converts the optical signals to AC electrical signals. In response to the electrical signals, the SLIC supplies AC signals of audio frequency, along with DC power from the battery, to the subscriber line. In such applications, where the SLIC and battery are remote from the central office, one battery or battery voltage generator may supply the battery voltage to only one or a few SLICs and their associated subscriber loops.
Newer generation chipsets are designed to operate in high density line card applications. The limited board space available in such line cards constrains the size of the package that is available for the devices like the SLIC and the SLAC. Another consideration that also reduces the size of the package is the desire to have lower per line cost for the line card. In general the smaller packages do not present a problem when the associated device does not generate significant heat. However for silicon devices, like the SLIC, that interface with the telephone line, the reduced silicon die size and the reduced package size coupled with the requirement of having to drive out heavy duty ringing signals to the telephony equipment present a challenging design problem with respect to heat generation.
In general, the ringing state is a challenging state in terms of power dissipation for the SLIC device. In a ringing state, the SLIC makes use of all of the available battery sources to drive out the maximum ringing signal. The rationale behind driving out the maximum possible ringing is that the SLIC needs to apply upwards of 40Vrms for the ringing signal at the longest loop (>1900 ohms) across a ringer load that is at least 5 REN. REN stands for Ringer Equivalent Number. It is a measurement of how much ringing power certain telephone equipment takes. REN numbers are used in the United States to designate how many pieces of telephony equipment can be connected to the same subscriber line and still get them ringing properly.
Because of the significant power requirements for generating high voltage AC ringing signals, the power supply for the line card must be sized to provide sufficient power to support the ringing signals. Generally, the peak power requirements for the line card are dictated based on the number of lines on the line card that can ring simultaneously. Sizing the power supply to support simultaneous ringing of all the subscriber lines, while being the most conservative approach, is also the most expensive.
Another alternative is to set a maximum limit for the number of lines that can ring simultaneously. Thus, the power requirements may be reduced and the power supply can be smaller. The line card will have sufficient power if the simultaneous ringing limit is never exceeded.
Typically ringing signals are cadenced, where a sequence of ringing intervals is separated by corresponding silence intervals. In theory, by interleaving the ringing intervals of multiple lines, the ringing intervals of some of the lines can be aligned with the silence intervals of other lines, to increase the effective number of lines that can ring simultaneously.
Ringing signal interleaving has been employed in the past to increase the effective ringing capacity of the line card. However, the ringing cadences were set at design time, and the interleaving was fixed. The lines on the line card were statically scheduled so that each line had an assigned timeslot for ringing. For example, in a typical cadence employed in the United States, a two second ringing interval is followed by a 4 second silence interval. This arrangement provides three timeslots for ringing, each ringing timeslot being shifted by two seconds. This approach has several limitations. For instance, when “distinctive ring” cadences are employed, the beginning portion or the even entire cadence does not follow the static 2 s ring/4 s silence pattern. Moreover, in some markets, such as the United Kingdom, a leading silence interval is provided prior to the first ringing interval to allow for caller ID data transmission. In the United States, the caller ID data is sent in the silence interval following the first ringing interval. In applications where such variable length ringing and silence intervals were possible, the static line scheduling approach cannot be used.
This section of this document is intended to introduce various aspects of art that may be related to various aspects of the disclosed subject matter described and/or claimed below. This section provides background information to facilitate a better understanding of the various aspects of the disclosed subject matter. It should be understood that the statements in this section of this document are to be read in this light, and not as admissions of prior art. The disclosed subject matter is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.